Vibrating flow meters such as, for example, densitometers and Coriolis flow meters are used for measuring a characteristic of flowing substances, such as, for example, density, mass flow rate, volume flow rate, totalized mass flow, temperature, and other information. Vibrating flow meters include one or more conduits, which may have a variety of shapes, such as, for example, straight, U-shaped, or irregular configurations.
The one or more conduits have a set of natural vibration modes, including, for example, simple bending, torsional, radial, and coupled modes. The one or more conduits are vibrated by at least one driver at a resonance frequency in one of these modes for purposes of determining a characteristic of the flowing substance. One or more meter electronics transmit a driver signal to the at least one driver, which is typically a magnet/coil combination, with the magnet typically being affixed to the conduit and the coil being affixed to a mounting structure or to another conduit. The driver signal causes the driver to vibrate the one or more conduits at the driver frequency in the driver mode. For example, the driver signal may be a periodic electrical current transmitted to the coil.
At least one pick-off detects the motion of the conduit(s) and generates a sinusoidal pick-off signal representative of the motion of the vibrating conduit(s). The pick-off is typically a magnet/coil combination, with the magnet typically being affixed to one conduit and the coil being affixed to a mounting structure or to another conduit. The pick-off signal is transmitted to the one or more electronics; and according to well known principals the pick-off signal may be used by the one or more electronics to determine a characteristic of the flowing substance or adjust the driver signal, if necessary.
Typically, vibrating flow meters are provided with two vibrating conduits that vibrate in opposition to each other in order to create an inherently balanced system. As a result, the vibrations from each conduit cancel each other out in a manner that prevents vibration or torque forces from being transmitted to any connecting structures. Likewise, when two vibrating conduits are used, vibrations of the mounting structure are canceled in the flow meter because the pick-offs generally measure only relative motion between the flow tubes, and externally induced vibrations tend to vibrate both tubes equally. There are, however, certain applications where dual conduits are undesirable, for example, due to problems with pressure drops or clogging. In such situations a single conduit system may be desirable.
However desirous a single conduit system may be, single conduit systems present inherent imbalance problems. Attempts at solving this problem have involved using a balancing structure, for example, a dummy tube or a balance bar, and using the motion of the balancing structure to balance out the system. Since, however, the overall mass of the tube, including the fluid within the tube, changes as the density of the fluid within the tube changes, these techniques by themselves have received limited success at eliminating imbalance problems.
FIG. 1 depicts a single conduit type vibrating flow meter according to the prior art. As shown, the flow meter includes a case 106 enclosing a balance bar 102. The balance bar 102 is cylindrical and encloses conduit 101. Conduit 101 has active portion 109 and inactive portions 110 and 110′, which are defined by the connecting rings 103, 104 of the balance bar 102. The inactive portions 110, 110′ extend beyond end elements 107, 108 of the case 106 to flanges (not shown). Conduit 101 has an input end 111 connected to an opening in case end 107 and an output end 112 connected to an opening in the case end 108.
In operation, conduit 101 and balance bar 102 are vibrated in phase opposition by a driver D. With substance flowing, the vibration of conduit 101 in this example induces a Coriolis response in conduit 101 that is detected by pick-off sensors LPO, RPO. The phase displacement between the pick-off sensors represents information pertaining to the flowing substance. The signal output of the velocity sensors is applied to meter electronics circuitry 125 via leads 122, 124 that processes the signals to derive the desired information pertaining to the flowing substance, such as for example a mass flow rate, a density, a viscosity, etc.
It is necessary that a vibrating flow meter provide accurate information over a wide range of operating conditions including substances of different density, temperature, and viscosity. In order to achieve this, it is desirable that the flow meter operate stably over a range of conditions. In order to achieve this stability, it is desirable for the flow meter vibrations to be isolated to the active conduit portion and balance system, because vibrations external to the vibratory system, whether induced by the vibrations of the flow meter or from another source, such as a pump, imposes additional accelerations on the flowing substance besides the Coriolis acceleration used to determine the fluid characteristics of the flowing substance. External vibration also repositions the nodes (area experiencing no motion) defining the active length of the conduit. This effect is difficult to compensate for and is subject to unknowable parameters such as the rigidity of the structure to which the meter is connected. Accordingly, undesired vibrations impede the ability of the flow meter to provide accurate output information regarding the flowing substance.
Prior art attempts at solving imbalance problems that arise due to changes in the density of the fluid involve adjusting the ratio of the vibration amplitude of the conduit relative to the vibration amplitude of the counterbalance structure. In balancing a structure, momentum is what is being balanced. Momentum is the product of mass and velocity, and velocity is proportional to vibration amplitude. Therefore, altering the vibration amplitude ratio alters meter balance. If, for example, the mass of a conduit (including the fluid located inside) and the mass of the counterbalance structure were initially equal and then the mass of the conduit were doubled (for example, as a result of a density increase in the fluid within the conduit), then reducing the amplitude of the conduit by half would restore balance to the conduit/counterbalance system. In practice, the combined amplitude of both the counterbalance structure and the conduit can be controlled by meter electronics. Accordingly, the conduit amplitude may be reduced to a lesser extent and the balance structure amplitude may be increased to some extent until in the above example, the ratio of the counterbalance amplitude relative to the conduit amplitude is 2:1.
The traditional method of adjusting the amplitude ratio as used in the prior art is to isolate the vibrating structure with a very soft (spring rate) mount. The idea is that a vibrating structure isolated in space is always balanced. For example if a spring joins two equal masses in space, such that when set vibrating out of phase with each other, the masses vibrate with equal amplitude and the spring has a motionless node half way between the masses. If one mass were to be increased and the masses were again set vibrating, the vibration amplitude of the increased mass would automatically decrease, and the vibration amplitude of the other mass would automatically increase to keep the momentum balanced. However, as a consequence, the new position of the node on the spring would relocate closer to the larger mass. The vibrating structure of a vibrating flow meter is similar, and node relocation is a problem.
Prior art flow meter designs that utilize self-balancing single tube meters are similar to a tuning fork wherein one tine is the active section of the flow tube, the other tine is the balance structure, and the handle is the inactive sections of the flow tube joining the active structure to the case. In this configuration, adding mass to one tine of the tuning fork decreases its amplitude and increases the amplitude of the other. The node, formerly at the junction of the two tines and the handle, relocates up the tine with the increased mass. The result is that the handle vibrates with the low-mass tine. If the vibrating handle is rigidly clamped, the vibration frequency rises, whereas if it is loosely clamped the frequency drops. This is a problem with flowmeters.
For the flow meter of FIG. 1, the vibrating system includes balance bar 102 and active conduit portion 109 which are vibrated in phase opposition. The ends of balance bar 102 and the conduit 101 are coupled by connecting rings 103, 104. Inactive tube portions 110, 110′ extend unsupported from the connecting rings 103, 104 to the case ends 107, 108. These inactive tube portions correspond to the tuning fork handle. They are necessary and they are unsupported because they are the soft mounts that enable the amplitude change with density. However, they vibrate like the tuning fork handle when the density of the fluid is changed. This is undesirable since the vibration may cause the vibration of the case 103 and flanges 106. Because the vibration amplitude of the case 103 and flanges 106 is dependent upon the stiffness of the structure to which the meter is mounted, error of unknown magnitude can be induced in the flow measurement.
Adjusting the amplitude ratio in traditional method has an additional drawback in prior art meters in that it results in the repositioning of the motionless nodes that reside along the axis of the vibrating structure. The area between the nodes defines the active length of the conduit. The active length affects the measurement sensitivity. If the nodes relocate outwardly towards the case ends, the active length increases. The formerly inactive tube sections bend as a part of the vibration and that bending motion imparts Coriolis acceleration to the fluid. The additional Coriolis acceleration either adds to or subtracts from the sensitivity of the flow meter. Because the rigidity with which the meter is attached to the pipeline affects the amount of the additional Coriolis acceleration, there is no way to compensate for the relocation of the nodes. This relocation of the nodes further degrades the measurement accuracy.
There is however, one form of node relocation that does not change the meter sensitivity. If the inactive portions of the flow tube are constrained to rotation about their axes, the nodes can move up and down the axes without changing the Coriolis acceleration of the fluid. This is because the so-called inactive tube portion has to bend in order to create a Coriolis acceleration in the fluid. No tube bending means no sensitivity change despite node relocation. Until now, however, this principle has not been used in Coriolis flow meters. Therefore, there is a need in the art for a system that can couple the flow tube to its case in such a manner that the tube is left free to rotate about its axis, but is substantially prevented from changing the active tube length. The present invention overcomes this and other problems and an advance in the art is achieved.